Electrolytic method for photoresist stripping

ABSTRACT

Photoresist stripping is performed by having a piecepart with a conductive layer that patterned by the photoresist immersed in a neutral solution. A voltage potential is applied to induce a current between the conductive layer and a counter electrode in neutral solution bath at a specified current density. After a short period of time, on the order of minutes, the photoresist is lifted off the piecepart. The piecepart is then removed from the bath, rinsed and dried.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for wet processing, and moreparticularly to a method for removing photoresists by electrostripping.Specifically, the present invention relates to a method of mechanicallyor physically lifting the photoresist off a surface by the generation ofgas bubbles.

2. Description of Related Art

Photoresists are widely used in electronics industry for patterndefinition on a conductive surface before electrolytic plating.Photoresists are light-sensitive compounds, which upon irradiation by asuitable light or other radiation source change their chemicalstructure. Organic polymer resist layers are usually applied in the formof either a dry film or liquid. If the resist is of a “negative” photodefined type, the underlayer is exposed to a pattern defining artworkand the unexposed portion of the resist is developed off. In a“positive” photo defined type, the exposed areas are rendered soluble ina developing solution. As an alternative, the resist may be definedusing a screen-printing process, or less commonly, using electron beamor laser ablation.

During the chemical process, such as plating, the photoresist isgenerally subject to structural and chemical modifications.Consequently, removal of the photoresist can present some problems.Also, the thermal stressing and ultraviolet hardening of the photoresistaffects the ability to remove the photoresist. Furthermore, the chemicalremoval of photoresists continues to pose environmental concerns.

Applications for photoresists include second level (board) and firstlevel (substrate) packaging as well as processing of semiconductorwafers and magnetic heads, among others. Stripping of photoresists maybe performed by using wet chemical processing or by a dry process suchas oxygen plasma etch. Wet chemical processes used for resist strippingmay involve water based strippers such as sodium or potassium hydroxide,tetramethyl ammonium hydroxide (TMAH) or organic strippers such asN-methyl-pyrrolidone (NMP). The resist stripping involves a chemicalattack of the stripper on the resist resulting in its surfacemodification, swelling, and at least some degree of dissolution in thestripper, which frequently leaves some residue. Stripping operations mayalso be aided by a spray or ultrasonic type agitation aimed at liftingthe chemically attacked resist.

When resist stripping is performed by chemical means, the interface bondbetween the resist and the underlying metal surface is chemicallybroken. Generally, sodium hydroxide in water chemically attacks theresist. However, the chemical bond is normally weaker than themechanical bond between the surfaces. It would be advantageous to employa process that specifically targets the mechanical bonds. It isdesirable to physically lift the photoresist from the underlyingsurface. It is also desirable to utilize the semiconductor wafer in-situas part of the electrolytic process.

In U.S. Pat. No. 5,676,760 issued to Aoki, et al., on Oct. 14, 1997,entitled “METHOD FOR WET PROCESSING OF A SEMICONDUCTOR SUBSTRATE,”electrolyzed waters, including an anode water and a cathode water, areapplied in a wet processing method for cleaning, etching, and rinsingprocessed semiconductor wafers. This process, however, is limited inthat electrostripping and physical lifting of the resist are notperformed.

In U.S. Pat. No. 4,968,398 issued to Ogasawara on Nov. 6, 1990, entitled“PROCESS FOR THE ELECTROLYTIC REMOVAL OF POLYIMIDE RESINS,” electrolysisis conducted using an exposed copper layer as a cathode on a polyimideresin and an insoluble anode to remove undesirable remaining polyimideresin from the substrate. Ogasawara, however, does not teach removing aphotoresist, or replacing a typical chemical stripping processes. In theOgasawara invention, photoresists are used to pattern a copper seedbefore copper electroplating, and stripped chemically from the coppersurface. Moreover, photoresists are used to pattern the polyimide andcopper surface before the polyimide etch step. Electrostripping of anyphotoresist is not taught or disclosed.

In U.S. Pat. No. 6,436,276 issued to Yakobson on Aug. 20, 2002, entitled“CATHODIC PHOTORESIST STRIPPING PROCESS,” a printed wiring board havingphotoresist on its surface is used as a cathode during electrolysis inan alkaline solution. However, Yakobson uses a solution that remainscapable of chemically resist stripping. In contrast, the presentinvention uses a solution that is not capable of chemical resiststripping. Furthermore, Yakobson separates the cathode and insolubleanode compartments by an ion selective membrane. No requirement isimposed by the present invention; the electrolytic cell is a singlecompartment cell.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide a method forelectrostripping photoresist in chemically inert solutions.

It is another object of the present invention to provide an electrolyticmethod for photoresist stripping by physically lifting the photoresistfrom a surface.

A further object of the invention is to provide an electrolytic methodfor photoresist stripping that replaces any chemical stripping processor chemical stripping solution.

It is yet another object of the present invention to provide anapparatus for photoresist stripping to physically lift the photoresistfrom a piecepart while utilizing the piecepart in the electrolyticprocess.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

SUMMARY OF THE INVENTION

The above and other objects, which will be apparent to those skilled inart, are achieved in the present invention, which is directed to amethod of stripping a photoresist from a piecepart having a conductivelayer under the photoresist, comprising: immersing the piecepart in aneutral solution; generating hydrogen gas bubbles in the neutralsolution for a period of time to lift the photoresist from thepiecepart; and removing the piecepart for rinsing and drying after thephotoresist is lifted. The method includes applying a voltage potentialacross the conductive layer and a counter electrode, while both are inthe neutral solution. The voltage potential is applied such that theconductive layer is a cathode and the counter electrode is an anode. Theneutral solution may include sodium citrate, sodium orthophosphate,sodium sulfate, sodium nitrate, ammonium acetate, sodium acetate, or awater-based ionic conductive chemical.

In a second aspect, the present invention is directed to a method ofstripping a photoresist from a piecepart having a conductive layer underthe photoresist, comprising: immersing the piecepart in a neutralsolution; immersing an inert counter electrode in the neutral solution;electrically applying a potential difference across the piecepart andthe counter electrode such that the piecepart acts as a cathode and thecounter electrode acts as an anode; generating hydrogen gas in theneutral solution to lift the photoresist off the piecepart; and removingthe piecepart from the solution for rinsing and drying. The piecepartmay comprise a circuit board, substrate, metal mask, semiconductorwafer, or magnetic head. The counter electrode may comprise conductivenon-corrosive material, such as stainless steel meshes or platinumcovered titanium. The step of generating hydrogen gas comprises inducinga current between the cathode and the anode. The neutral solutionremains incapable by itself of stripping resist. The photoresistincludes Riston 4840. The photoresist may have a thickness of 2 mils inone embodiment with an induced current density on the order of 50mA/cm2, or a thickness of 4 mils in a second embodiment with an inducedcurrent on the order of 100 mA/cm2. In the first embodiment thepiecepart may be immersed in the neutral solution at a concentration onthe order of 50 g/l at room temperature. In the second embodiment, thepiecepart may be immersed in the neutral solution at a concentration onthe order of 150 g/l at 30° C. The method further includes maintainingthe piecepart in the neutral solution with the voltage potential appliedfor approximately three to fifteen minutes.

In a third aspect, the present invention is directed to an apparatus forelectrolytic stripping of a photoresist from a piecepart having aconductive layer under the photoresist, comprising: an electrolytic cellcontaining a neutral water-based ionic conductive solution, thepiecepart, and an electrode; and a power supply having a negativeterminal electrically attached to the conductive layer and a positiveterminal electrically attached to the electrode, and generating apotential difference between the conductive layer and the electrode;such that hydrogen gas is generated in the solution when the potentialdifference is applied.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figure is for illustration purposes only and is notdrawn to scale. The invention itself, however, both as to organizationand method of operation, may best be understood by reference to thedetailed description which follows taken in conjunction with theaccompanying drawing, in which FIG. 1 depicts an electrolytic cell forelectrolytic stripping of a photoresist.

DESCRIPTION OF THE PREFERRED EMBODIMENTS(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIG. 1 of the drawings in which likenumerals refer to like features of the invention.

FIG. 1 depicts an apparatus for stripping a photoresist. An electrolyticcell 5 is used for electrolytic stripping of a photoresist. A piecepart12 is placed in a tank 10 containing a bath of neutral electrolyticsolution 14. The piecepart 12 includes photoresist material 16 on aconductive surface 18. Also immersed in the bath is a counter electrode20. A voltage potential is applied across the conductive surface 18 andthe counter electrode 20 by a power supply 22. Photoresist strippingfrom the conductive surface 18 is performed in the electrolytic cell 5with the neutral electrolyte 14, such as sodium citrate, sodiumorthophosphate, sodium sulfate, sodium nitrate, ammonium acetate, sodiumacetate, or with other non-aggressive, water based ionic conductivechemical. Importantly, a chemical attack on the resist is not reliedupon to remove the resist from the piecepart. Rather, the mechanicalbond is broken between the resist and the conductive surface by theorigination of hydrogen gas. More solution is able to get between theresist and the conductive metal surface where hydrogen gas bubblescontinue to be formed, lifting the resist away from the conductivesurface. Hydroxide ions generated in-situ during hydrogen gas evolutionaid in dislodging of the resist.

The piecepart 12 may be a circuit board, substrate, metal mask,semiconductor wafer, magnetic bead, and the like. When immersed in thetank with a voltage potential applied, it acts as a cathode. Theconductive metal on the piecepart is held at a negative potential. Theinsoluble counter electrode 20 may include such material as stainlesssteel mesh, platinum covered titanium, or the like, and is used as theanode. As discussed above, the solution is preferably, a neutral,inorganic salt, capable of carrying a current without decomposition orchemical attack on the piecepart. Importantly, this technique allows thesolution to be non-toxic, and thus more environmentally friendly fordisposal. The applied voltage potential causes a current to pass throughthe cell. It is the decomposition of water at the cathode that resultsin generation of hydrogen gas bubbles around the photoresist and at aninterface between the photoresist and the conductive surface. Thehydrogen bubbles mechanically lift the resist from the conductivesurface. In addition, the pH of the electrolyte, in close vicinity ofthe conductive surface, becomes alkaline due to waterdecomposition/hydrogen gas evolution. This in-situ increase in thesolution's pH aids in the resist stripping by attacking the interfacebetween the resist and a conductive surface upon which the resist wasdeposited. As the resist stripping and hydrogen gas evolution takesplace at the cathode, oxygen is generated at the insoluble anode. Thestripped resist is not chemically degraded. It retains its originalshape and pattern. When the stripping is completed, the resist floats tothe top surface of the solution and is easily removed by filtering. Thepiecepart is then removed from the solution, rinsed and dried.

The following two embodiments illustrate the invention:

In a first preferred embodiment, a metal mask is patterned with aphotoresist such as Riston 4840, generally on the order of 2 mils thick,and immersed in a sodium citrate solution at a concentration of 50 g/lat room temperature. A voltage potential is applied to induce a currentbetween the mask and a stainless steel counter electrode (anode) at acurrent density of approximately 50 mA/cm². At these conditions,complete lifting of the resist from the mask may be achieved in aboutthree minutes.

In a second embodiment, a semiconductor wafer with a conductive copperlayer is patterned with a photoresist, such as Riston 4840, generally onthe order of 4 mils thick, and electroplated. Thereafter, the wafer isimmersed in electrolytic cell containing 150 g/l of sodium sulfatesolution at 30° C. A voltage potential is applied to induce a currentbetween the semiconductor wafer and the counter electrode at a currentdensity of approximately 100 mA/cm². Current is passed through the cellfor approximately seven minutes, resulting in a complete lifting of theresist from the wafer.

Typically, current densities may be adjusted to lift all of the resistfrom the piecepart in about three to fifteen minutes. Once the resist islifted, the piecepart is removed from the solution, rinsed and dried.

1. A method of stripping a photoresist from a piecepart having aconductive layer under said photoresist, comprising: immersing saidpiecepart in a neutral solution; generating hydrogen gas bubbles in saidneutral solution for a period of time to lift said photoresist from saidpiecepart; and removing said piecepart for rinsing and drying after saidphotoresist is lifted.
 2. The method of claim 1 including applying avoltage potential across said conductive layer and a counter electrode,while both are in said neutral solution.
 3. The method of claim 2including applying said voltage potential such that said conductivelayer is a cathode and said counter electrode is an anode.
 4. The methodof claim 1 including said neutral solution of sodium citrate, sodiumorthophosphate, sodium sulfate, sodium nitrate, ammonium acetate, sodiumacetate, or a water-based ionic conductive chemical.
 5. A method ofstripping a photoresist from a piecepart having a conductive layer undersaid photoresist, comprising: immersing said piecepart in a neutralsolution; immersing an inert counter electrode in said neutral solution;electrically applying a potential difference across said piecepart andsaid counter electrode such that said piecepart acts as a cathode andsaid counter electrode acts as an anode; generating hydrogen gas in saidneutral solution to lift said photoresist off said piecepart; andremoving said piecepart from said solution for rinsing and drying. 6.The method of claim 5 wherein said piecepart includes a circuit board,substrate, metal mask, semiconductor wafer, or magnetic head.
 7. Themethod of claim 5 including said neutral solution of sodium citrate,sodium orthophosphate, sodium sulfate, sodium nitrate, ammonium acetate,sodium acetate, or a water-based ionic conductive chemical.
 8. Themethod of claim 5 including said counter electrode comprising aconductive non-corrosive material.
 9. The method of claim 8 wherein saidcounter electrode includes stainless steel meshes or platinum coveredtitanium.
 10. The method of claim 5 wherein said step of generatinghydrogen gas comprises inducing a current between said cathode and saidanode.
 11. The method of claim 5 including a neutral solution incapableof stripping resist.
 12. The method of claim 10 wherein said photoresistincludes Riston
 4840. 13. The method of claim 12 including saidphotoresist having a thickness of 2 mils.
 14. The method of claim 12including said photoresist having a thickness on the order of 4 mils.15. The method of claim 13 including inducing said current having acurrent density on the order of 50 mA/cm².
 16. The method of claim 15including immersing said piecepart in said neutral solution at aconcentration on the order of 50 g/l at room temperature.
 17. The methodof claim 14 including inducing said current having a current density onthe order of 100 mA/cm².
 18. The method of claim 17 including immersingsaid piecepart in said neutral solution at a concentration on the orderof 150 g/l at 30° C.
 19. The method of claim 5 including maintainingsaid piecepart in said neutral solution with said voltage potentialapplied for approximately three to fifteen minutes.
 20. An apparatus forelectrolytic stripping of a photoresist from a piecepart having aconductive layer under said photoresist, comprising: an electrolyticcell containing a neutral water-based ionic conductive solution, saidpiecepart, and an electrode; and a power supply having a negativeterminal electrically attached to said conductive layer and a positiveterminal electrically attached to said electrode, and generating apotential difference between said conductive layer and said electrode;such that hydrogen gas is generated in said solution when said potentialdifference is applied.